Cellulose is the most abundant structural biopolymer. All forms of plant life contain cellulose. Because of its nearly ubiquitous distribution in nature and human kind's long exposure to cellulose, cellulose and its derivatives are generally recognized as the safest and most acceptable polymer class for use in food and pharmaceutical products.
Cellulose is a solid natural carbohydrate polymer (polysaccharide) composed of anhydroglucose units (β-D glucopyranose rings) joined by an oxygen linkage (β-1,4-glycosidic linkage) and has the empirical formula (C6H10O5)n. Cellulose is insoluble in water and organic solvents. It will swell in sodium hydroxide solutions and is soluble in Schweitzer's reagent. Cellulose exists in three forms—α, β, and γ. α-cellulose has the highest degree of polymerization and is the chief constituent of paper pulp. It is insoluble in strong sodium hydroxide solution. The β and γ forms have much lower DP and are known as hemicellulose. Cellulose can be decomposed to glucose by the enzyme cellulase or by hydrolysis.
Cellulose is a complex composite material which structurally comprises three hierarchical levels: (i) The molecular level of the single molecule; (ii) the supermolecular level concerning the packing and aggregation of the molecules in crystals called microfibrils; and (iii) the morphological level, i.e., the arrangement of microfibrils and interstitial voids in relation to the cell wall. On the molecular level, the linear chains of glucose units form whisker-like crystals which are assembled into the superstructure. The structural organization at all levels influences the macroscopic properties of the material and is equally of importance for the chemical reactions taking place during processing.
The “classical” model of cellulose, however, is two-phase, assuming a composite arrangement of distinct crystalline and extended amorphous regions (H. Krssig, Cellulose: Structure, Accessibility and Reactivity; Polymer Monographs 11, Gordon and Breach Science Publ.: Yverdon 1993). Concepts like crystallinity and amorphicity have been used to describe homogeneous states of matter such as in the “classical” cellulose model. (These concepts can be, however, rather ill-defined when it comes to treat dense composite materials like cellulose given that intermolecular correlations do not build up or die off abruptly at some fictitious interfaces.) Depending on the degree of order of arrangement and hydrogen bonding between cellulose chains, the crystallinity of cellulose may range from 50% to 90%. The crystallinity of native cellulose is about 70% (P. H. Hermans and A. Weidinger, J. Poly. Sci., IV, 135 (1949)).
Chemical reagents react with or penetrate the amorphous regions much more readily than the crystalline regions. Depolymerization of cellulose by acid or enzyme hydrolysis is limited by the degree of crystallization. The amorphous and crystalline regions in cellulose fibers behave differently in most chemical reactions such as dyeing, swelling, and oxidation. Therefore, it is often of interest to determine the crystalline fraction of cellulose or process cellulose to alter the structure to make it more amorphous.
The reactions of cellulose with mineral acids to prepare non-fibrous, low molecular weight (i.e., low degree of polymerization) cellulose products suitable for use in food, cosmetics, pharmaceutical, and like products, have been studied. The reactivity of cellulose towards acids depends on the crystallinity of the cellulose source, acid concentration, and the reaction temperature and duration.
There are modified celluloses and “amorphous” celluloses. Microcrystalline cellulose (MCC) is one form of modified cellulose. The “amorphous” cellulose known to this point is cellulose chemically bound to another organic substance. An example is carboxymethylcellulose (CMC). Phosphoric acid swollen cellulose (PASC) is also known. PASC is produced by swelling MCC in concentrated phosphoric acid; though often described as amorphous, it is probably a low-crystallinity form of cellulose 11. Atalla, R. H.1993. The structures of native celluloses, p. 25-39. In P. Suominen, and T. Reinikainen (ed.), Trichoderma reesei cellulases and other hydrolases. Foundation for Biotechnical and Industrial Fermentation, Helsinki, Finland.